Wind turbines are now frequently being installed as an alternative to conventional power plants that use for example fossil fuels for energy production. Onshore or offshore wind parks may include a number of wind turbines each capable of producing electric power from one to several megawatt. Wind turbines may thus make a substantial contribution to electric power generation. A wind turbine generally comprises a tower on the top of which nacelle is rotatably mounted. A yaw drive may be provided for directing the wind turbine into the wind. It further comprises a rotor including a hub to which one or several rotor blades are mounted, e.g. 3, a generator generating electric energy upon rotation of the rotor and a transformer for transforming the generated electric energy such that it may be fed into the power grid. The wind turbine may of course comprise further components, such as a gear box, inverters, control electronics and the like.
For conversion of wind energy into electrical energy, the moving air rotates the hub via the aerodynamic effect of the rotor blade, the hub turning a rotor of the generator directly or via a gear box. The electrical energy is converted to fit the conditions on the power grid by means of the transformer and a converter unit.
The nacelle of the wind turbine thus comprises several components that may require servicing after a certain period of operation. During service and in the case of a repair, spare parts have to be handled. As most parts are too heavy to be lifted by hand, cranes and lifting devices are used.
For lifting spare parts or tools from the base of the wind turbine tower (e.g. from a base platform, the ground or a vessel/vehicle carrying the parts) up to the nacelle, it is known to mount a crane on top of the nacelle that may perform a lifting operation. Weather conditions, such as strong winds may prevent the operation of such cranes as the load may start to swing. This is a particular problem for offshore wind turbines, where strong winds are often present. By waiting for suitable weather conditions, the servicing of the wind turbine may be delayed, resulting in a prolonged down time of the wind turbine and thus increased expenses or loss of revenue. Also, the crane first has to be brought up to the nacelle and mounted thereto, which is time and labor intensive.
Some nacelles may comprise a platform on which equipment, such as spare parts, may be landed by means of an helicopter (helihoist platform). An external crane may be used for lifting equipment from the helihoist platform into the nacelle.
The equipment, in particular spare parts may need to be positioned inside the nacelle, for which an additional crane is required that transports each part to the location where it is to be mounted. Several cranes and several lifting steps are thus needed.